The disclosure proceeds from known ball seat or needle valves for adjusting a throughflow of fluid media. Such valves with a spherical or needle-shaped closing element are used in many fields of technology in which a throughflow of fluid media, such as for example gases or liquids, must be adjusted, for example in the field of hydraulic regulating systems.
An important application example of such ball seat or needle valves can be found in the field of automotive engineering, in particular in the field of injection technology. Such valves are used in numerous injection devices for hydraulic pressure regulation and/or control of the injection behavior of such systems. In particular in the field of high-pressure accumulator injection systems (common rail system), these valves are used to control the stroke of an injection valve member which opens or closes injection openings. Examples of such devices are specified in DE 101 52 173 A1 and DE 196 50 865 A1. Here, a control chamber which directly or indirectly influences the stroke of the injection valve member is connected to the valve directly or via a further bore via an inlet. Aside from the closing element, the valve has an actuator which presses the closing element into a valve seat or raises the closing element from said valve seat in order to separate the control chamber from a relief chamber, or connect said control chamber to the relief chamber.
As presented for example in DE 101 52 173 A1, valves for adjusting throughflow known from the prior art generally have a prethrottle on that side of the inlet which faces toward the control chamber. Said prethrottle is adjoined in the direction of the valve by one or more widenings of the inlet, which may take different forms. The flow through the prethrottle is generally configured so as to be lower than the volume flow which passes between the valve seat and the closing element when the ball seat valve or needle valve is fully open. As a result of the prethrottle, the throughflow is substantially independent of the closing element stroke tolerance to be set.
In practice, however, conventional valves known from the prior art which have a spherical or needle-shaped closing element have the disadvantage of intense erosion. If there is a large pressure gradient between the entry and exit of a throttle bore, such as is often the case in throttle bores in high-pressure injection valves which operate with pressures of up to 3500 bar, the medium accelerates in the throttle bore so intensely that the pressure falls to approximately 0 bar, and cavitation bubbles form. To prevent said cavitation bubbles from imploding on the surface of a component, or even on the valve seat, and thereby damaging the surface, it has proven to be advantageous for a large amount of the cavitation bubbles to be converted in a controlled manner into liquid phase by means of a second prethrottle, and for damage to functionally critical components to thereby be prevented, as described in DE 10 2007 004 553 A1.
The flow speed of the medium increases intensely in the inlet region of a throttle bore. At the same time, the pressure in the medium falls. If the pressure falls to the vapor pressure of the medium, cavitation bubbles form. Since the pressure can fall no further below the vapor pressure of the medium, the mass flow through the throttle bore remains constant after the vapor pressure is reached. The vapor-filled cavitation bubbles take up a larger volume than the liquid medium. This results in a pressure recovery in the onward flow through the throttle bore. The pressure recovery is further intensified by the contact of the flow against the throttle wall. In throttles with relatively high pressure recovery, a higher back pressure can be applied downstream of the throttle before flow no longer passes in a cavitating manner through the throttle. The greater the pressure recovery, the more independent the throttle is of the conditions downstream of the throttle.
To make throttle bores as insensitive to pressure fluctuations and geometric influences as possible, a cavitating throughflow is desirable. Ideally, the cavitation transition point should lie as close as possible to the start of the throttle, because the attainable pressure recovery is greater and therefore the tendency for a cavitating flow through the throttle increases. In contrast, if the cavitation transition point is situated at the end of the throttle bore in the region of the throttle outlet, the pressure recovery can take place only in a diffuser bore. Since the latter however has a relatively large cross section, the attainable pressure recovery is lower. If the throttle duct widens slowly in the flow direction, then the cavitation transition point lies at the start of the throttle, because the narrowest cross section of the throttle duct lies in the vicinity of the throttle inlet.
Throttle bores which are presented in the fuel injectors according to the prior art may be produced for example by means of cutting production processes such as drilling. It is however also possible for throttle bores to be produced by erosion. In the case of erosion, as a result of the more intense burn-off of the electrode at the tip, a throttle bore is generated which tapers slowly in the erosion direction. The tapering of the throttle bore in the erosion direction takes place on the micrometer scale. An eroded throttle can accordingly have the cavitation transition point in the region of the inlet only if flow passes through said throttle counter to the erosion direction. In contrast, if erosion counter to the throughflow direction is not possible or uneconomical or if there are other reasons against this, a throttle duct is formed which tapers slowly in the throughflow direction. As a result, a cavitation transition point is generated a short distance upstream of the throttle outlet, which leads to the abovementioned reduced maximum attainable pressure recovery.
To produce a throttle bore which, contrary to the “natural” erosion process, widens slowly in the erosion direction, the approach at present is to set the component to be eroded in a tumbling motion such that the origin of the tumbling motion is situated in the vicinity of the throttle inlet. This hitherto practiced method is however associated with the disadvantage that small throttle throughflow tolerances can be realized only with difficulty.